2012ApJ...750..165R

We combine H I 21 cm observations of the Milky Way, M31, and the local galaxy population with QSO absorption-line measurements to geometrically model the three-dimensional distribution of infalling neutral-gas clouds ("high-velocity clouds" (HVCs)) in the extended halos of low-redshift galaxies. We demonstrate that the observed distribution of HVCs around the Milky Way and M31 can be modeled by a radial exponential decline of the mean H I volume-filling factor in their halos. Our model suggests a characteristic radial extent of HVCs of R_halo∼ 50 kpc, a total H I mass in HVCs of ∼10⁸ M_☉, and a neutral-gas accretion rate of ∼0.7 M_☉/yr for M31/Milky-Way-type galaxies. Using a Holmberg-like luminosity scaling of the halo size of galaxies we estimate R_halo∼ 110 kpc for the most massive galaxies. The total absorption cross-section of HVCs at z ~ 0 most likely is dominated by galaxies with total H I masses between 10^8.5 and 10¹⁰ M_☉. Our model indicates that the H I disks of galaxies and their surrounding HVC population can account for 30%-100% of intervening QSO absorption-line systems with log N(H I) ≥ 17.5 at z ~ 0. We estimate that the neutral-gas accretion rate density of galaxies at low redshift from infalling HVCs is dM_HI/dt/dV ≈ 0.022 M_☉/yr/Mpc3, which is close to the measured star formation rate density in the local universe. HVCs thus may play an important role in the ongoing formation and evolution of galaxies.